1. Field of the Invention
The present invention relates to nanoelectronic devices, and in particular to nanostructured sensor systems for measurement of analytes, for example by measurement of variations of capacitance, impedance or other electrical properties of nanostructure elements in response to an analyte.
2. Description of Related Art
Nanowires and nanotubes, by virtue of their small size, large surface area, and near one-dimensionality of electronic transport, are promising candidates for electronic detection of chemical and biological species. Field effect transistors (“FET”) fabricated from component semiconducting single wall carbon nanotubes (“NT”) have been studied extensively for their potential as sensors. A number of properties of these devices have been identified, and different mechanisms have been proposed to describe their sensing behavior. Devices that incorporate carbon nanotubes have been found to be sensitive to various gases, such as oxygen and ammonia, and these observations have confirmed the notion that such devices can operate as sensitive chemical sensors.
Nanotubes were first reported in 1993 by S. Iijima and have been the subject of intense research since. Single walled nanotubes (“SWNT”) are characterized by strong covalent bonding, a unique one-dimensional structure, and exceptionally high tensile strength, high resilience, metallic to semiconducting electronic properties, high current carrying capacity, and extreme sensitivity to perturbations caused by charged species in proximity to the nanotube surface.
SWNT devices, including FETs and resistors, can be fabricated using nanotubes grown on silicon or other substrates by chemical vapor deposition from iron-containing catalyst nanoparticles with methane/hydrogen gas mixture at 900 degrees C. Other catalyst materials and gas mixtures can be used to grow nanotubes on substrates, and other electrode materials and nanostructure configurations and have been described previously by Gabriel et al. in U.S. patent application Ser. No. 10/099,664 and in U.S. patent application Ser. No. 10/177,929, both of which are incorporated by reference herein. Currently, technology for constructing practical nanostructure devices is in its infancy. While nanotube structures show promise for use as sensor devices and transistors, current technology is limited in many ways.
For example, it is desirable to take advantage of the small size and sensitivity of nanotube and other nanostructure sensors to sense biological molecules, such as proteins. But a useful sensor of this type should selectively and reliably respond to a molecular target of a specific type. For example, it may be desirable to selectively sense a specific protein, while not responding to the presence of other proteins in the sample. Examples of covalent chemical attachment of biological molecules to nanotubes, including proteins and DNA, are known in the art, although it has not been convincingly demonstrated that useful detection of specific proteins or other large biomolecules can be accomplished in this way. For one thing, covalent chemical attachment has the disadvantage of impairing physical properties of carbon nanotubes, making structures of this type less useful as practical sensors. In addition, carbon nanotubes are hydrophobic, and generally non-selective in reacting with biomolecules.
It is desirable, therefore, to provide a nanotube sensing device that is biocompatible and exhibits a high degree of selectivity to particular targets. As described in commonly assigned patents and applications incorporated by reference herein, nanoelectronic sensors having active elements comprising nanostructures offer salient advantages for analyte detection for a wide scope of applications, including industrial, medical and biomolecular sensing.